1. Field of the Invention
The invention relates to a power tool having a torque transmission device that transmits torque and interrupts torque transmission between an input shaft and an output shaft, and more particularly, to a mechanical torque transmission device that transmits torque by engagement of a projection and a recess.
2. Description of the Related Art
Japanese laid-open patent publication No. 51-111550 discloses a torque transmission device provided as a torque limiter (an overload protection device) that protects a machine from excessive torque developed for any reason during torque transmission, or particularly, a mechanical torque transmission device that transmits torque by engagement of a projection and a recess.
In the above-described torque transmission device, a projection is formed on one of clutch surfaces (engagement surfaces) of a driving-side disk clutch and a driven-side disk clutch which are coaxially arranged and a recess is formed in the other clutch surface, and torque transmission is effected by engagement of the projection and the recess with each other and interrupted by disengagement of the projection and the recess from each other. The projection-side and recess-side engagement surfaces which are formed as torque transmitting surfaces on the projection and the recess are inclined at a predetermined angle in a direction of a rotational axis and linearly extend along normals of the disk clutches in radial directions.
The projection and the recess of the torque limiter are engaged with each other by spring force which defines maximum transmitting force. When overload torque is developed, the projection and the recess are caused to slide with respect to each other in an axial direction against spring force by axial force which acts between the projection-side and recess-side engagement surfaces (inclined surfaces in the direction of the rotational axis), so that the projection and the recess are disengaged from each other. Particularly, when the torque transmission device is used as the torque limiter, the engagement surfaces are disengaged from each other while being subjected to heavy load, so that the engagement surfaces easily wear. In this point, further improvement is desired.